If I understand correctly, you're saying: get the most common screen resolutions (let's say they're 800 x 600 and 1080 x 1920) and report whichever is the smallest that is still larger than the current letterboxed resolution?
I suppose the goal of this is to make the screen resolution report something more common for people using RFP so they look a little less unusual? ("Who's this person with a screen resolution of 620 x 960?")
However, what do we do for extremely high or low resolution situations? Especially when one of those is in one dimension? e.g. if your letterbox size is 5000px x 800px? Or 300px x 75px?